During combustion of fuels that contain sulfur compounds, oxides of sulfur (SOX), such as sulfur dioxide (SO2), and sulfur trioxide (SO3) are produced as a result of oxidation of the sulfur. Some fuels may contain nitrogen compounds that contribute to the formation of oxides of nitrogen (NOX) which are primarily formed at high temperatures by the reaction of nitrogen and oxygen from the air used for the reaction with the fuel, such as nitric oxide (NO) and nitrogen dioxide (NO2) or its dimer (N2O4). Many technologies have been developed for reduction of SOX and NOX, but few can remove both types of pollutants simultaneously in a dry process or reliably achieve cost effective levels of reduction.
Typical sources of nitrogen and sulfur oxide pollutants are power plant stack gases, automobile exhaust gases, heating-plant stack gases, and emissions from various industrial process, such as smelting operations and nitric and sulfuric acid plants. Power plant emissions represent an especially formidable source of nitrogen oxides and sulfur oxides, by virtue of the very large tonnage of these pollutants and such emissions discharged into the atmosphere annually. Moreover, because of the low concentration of the pollutants in such emissions, typically 500 ppm or less for nitrogen oxides and 3,000 ppm or less for sulfur dioxide, their removal is difficult because very large volumes of gas must be treated.
In the past to meet the regulatory requirements, coal-burning power plants have often employed a wet scrubbing process, which commonly uses calcium compounds to react with SOX to form gypsum. This waste product is normally discarded as a voluminous liquid slurry in an impoundment and ultimately is capped with a clay barrier, which is then covered with topsoil once the slurry is de-watered over time. Alternatively, some power-plant operators have chosen to burn coal that contains much lower amounts of sulfur to reduce the quantities of SOX emitted to the atmosphere. In the case of NOX, operators often choose to decrease the temperature at which the coal is burned. This in turn decreases the amount of NOX produced and therefore emitted; however, low temperature combustion does not utilize the full heating value of the coal, resulting in loss of efficiency.
Turbine plants normally use natural gas, which contains little or no sulfur compounds, to power the turbines, and therefore virtually no SOX is emitted. On the other hand at the temperature that the turbines are commonly operated, substantial NOX is produced. In addition to Selective Catalytic Reduction (SCR) processes for conversion of NOX to nitrogen, water vapor, and oxygen, which can be safely discharged, some operators choose to reduce the temperature at which the turbines are operated and thereby reduce the amount of NOX emitted. With lower temperatures the full combustion/heating value of natural gas is not realized, resulting in loss of efficiency. Unfortunately for these operators, newer environmental regulation will require even greater reduction of SOX and NOX emissions necessitating newer or more effective removal technologies and/or further reductions in efficiency.
Operators of older coal-burning power plants are often running out of space to dispose of solid wastes associated with use of scrubbers that use calcium compounds to form gypsum. Operators of newer plants would choose to eliminate the problem from the outset if the technology were available. Additionally, all power plants, new and old, are faced with upcoming technology requirements of further reducing emissions of NOX and will have to address this issue in the near future. Thus, plants that currently meet the requirements for SOX emissions are facing stricter requirements for reduction of NOX for which there has been little or no economically feasible technology available.
Of the few practical systems, which have hitherto been proposed for the removal of nitrogen oxides from power plant flue gases, all have certain disadvantages. Various methods have been proposed for the removal of sulfur dioxide from power plant flue gases, but they too have disadvantages. For example, wet scrubbing systems based on aqueous alkaline materials, such as solutions of sodium carbonate or sodium sulfite, or slurries of magnesia, lime or limestone, usually necessitate cooling the flue gas to about 55° C. in order to establish a water phase. At these temperatures, the treated gas requires reheating in order to develop enough buoyancy to obtain an adequate plume rise from the stack. U.S. Pat. No. 4,369,167 teaches removing pollutant gases and trace metals with a lime slurry. A wet scrubbing method using a limestone solution is described in U.S. Pat. No. 5,199,263.
Considerable work has also been done in an attempt to reduce NOX pollutants by the addition of combustion catalysts, usually organo-metallic compounds, to the fuel during combustion. However, the results of such attempts have been less successful than staged combustion. NOX oxidation to N2 is facilitated by ammonia, methane, et al. which is not effected by SOX is described in U.S. Pat. No. 4,112,053. U.S. Pat. No. 4,500,281 teaches the limitations of organo-metallic catalysts for NOX removal versus staged combustion. Heavy metal sulfide with ammonia is described for reducing NOX in stack gases in U.S. Pat. No. 3,981,971.
Considerable effort has been expended to remove sulfur from normally solid fuels, such as coal, lignite, etc. Such processes include wet scrubbing of stack gases from coal-fired burners. However, such systems are capital intensive and the disposal of wet sulfite sludge, which is produced as a result of such scrubbing techniques, is also a problem. Cost inefficiencies result from the often-large differential pressures across a wet scrubber removal system; differential pressures in excess of 30 inches of water column (WC) are not unusual. Also, the flue gases must be reheated after scrubbing in order to send them up the stack, thus reducing the efficiency of the system. Both U.S. Pat. Nos. 4,102,982 and 5,366,710 describe the wet scrubbing of SOX and NOX.
The combustion gas stream from a coal-burning power plant is also a major source of airborne acid gases, fly ash, mercury compounds, and elemental mercury in vapor form. Coal contains various sulfides, including mercury sulfide. Mercury sulfide reacts to form elemental mercury and SOX in the combustion boiler. At the same time other sulfides are oxidized to SOX and the nitrogen in the combustion air is oxidized to NOX. Downstream of the boiler, in the ducts and stack of the combustion system, and then in the atmosphere, part of the elemental mercury is re-oxidized to form mercury compounds, primarily to mercuric chloride (HgCl2). This occurs by reactions with chloride ions or the like normally present in combustion reaction gases flowing through, the combustion system of a coal-burning power plant.
Many power plants emit daily amounts of up to a pound of mercury, as elemental mercury and mercury compounds. The concentration of mercury in the stream of combustion gas is about 4.7 parts per billion (ppb) or 0.0047 parts per million (ppm). Past efforts to remove mercury from the stream of combustion gas, before it leaves the stack of a power plant, include: (a) injection, into the combustion gas stream, of activated carbon particles or particulate sodium sulfide or activated alumina without sulfur; and (b) flowing the combustion gas stream through a bed of activated particles.
In addition to SOX, NOX, mercury (elemental and compound), combustion, industrial and other process gases may contain other pollutants of concern such as hydrogen sulfide, totally reduced sulfides (TRS), oxides of carbon, and ash to name a few. EnviroScrub Technologies Corporation has developed pollutant removal systems and processes that overcome many of the shortcomings of prior art pollutant removal systems utilizing a regenerable sorbent of oxides of manganese. Some of these systems and process are referred to and commonly known as Pahlman™ systems and processes and are described in U.S. published application Ser. No. 20020150516, and international published application numbers WO0228513A2, WO0209852A2, and WO0209852A3, the disclosures of which are incorporated herein by this reference. They also include processes for regeneration of the sorbent and for recovery and/or production of useful values and by-products from regeneration processing streams.
Applicants have now recognized and determined that different metal oxides may be selected and utilized as the sorbent based upon certain physical or chemical properties of the metal oxides themselves and/or of the reaction products formed between the metal oxides and the target pollutants in gas streams.
During regeneration, reacted sorbent may be rinsed or washed to remove reaction products from the sorbent surface. Ash and other spectator particulates can accumulate in the rinses, washes or other processing solutions and streams during regeneration. They can interfere with or inhibit sorbent regeneration, by-product production and recovery of useful values present in the processing solutions and streams. Their presence may increase to undesired levels through several cycles and eventually require disposal or other costly treatment and purchases for processing solutions. Further, the overall rate or efficiency of regeneration, production or recovery may be reduced at significant levels. Applicants have found that with dry or wet magnetic separation techniques, non-magnetic or weakly magnetic particulate, spectator particulates, can be separated from certain metal oxides utilized as sorbents. Removal of ash and other spectator particulates prevents their accumulation in regeneration processing streams avoiding potential costs or deleterious affects during regeneration or recovery.